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The effect of sodium halide salts on the kinetics of tetrahydrofuran hydrate formation by using a differential scanning calorimetry method
Journal of Molecular Liquids 292 (2019) 111279
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
The effect of sodium halide salts on the kinetics of tetrahydrofuran hydrate formation by using a differential scanning calorimetry method Elnaz Hoseinynezhad, Farshad Varaminian ⁎ Faculty of Chemical, Gas and Petroleum Engineering, Semnan University, Semnan, Iran
a r t i c l e
i n f o
Article history: Received 26 December 2018 Received in revised form 16 June 2019 Accepted 30 June 2019 Available online 2 July 2019 Keywords: Sodium halides Tetrahydrofuran Differential scanning calorimetry Formation kinetics Gas hydrate
a b s t r a c t In this study, the effect of sodium halide salts including Sodium Fluoride, Sodium Iodide, Sodium Bromide, and Sodium Chloride was investigated on the kinetics of tetrahydrofuran hydrate formation by using a differential scanning calorimetry setup. It was found that the anions of salts can destroy hydrogen bonding between the water molecules and prevent the growth of gas hydrate. The fluorine with the smallest ionic radius and the highest enthalpy of hydration was the most effective material to decrease the hydrate growth rate. According to the results, the formation kinetics of hydrate changes with the concentration of salts in a different manner. Results show that the hydrate formation rate decreases with increasing the concentration of salts up to 0.6 wt%, while it increases when the concentration is at a range of 0.6 to 1.2 wt%. The rate begins to decrease by the concentration of N1.2 wt%. It was found that the parameters such as the hydration effect and the activity coefficients of salt solutions can affect the kinetics of tetrahydrofuran hydrate formation. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Natural gas hydrates are known as one of the most effective alternatives to oil resources [1–3]. Governmental agencies at around the world have supported the study and development of the hydrate resources so that they could provide a great achievement in theoretical researches, applied techniques and equipment innovations. Natural gas hydrate is a crystalline compound that formed through water and gas molecules at the specific conditions (low temperatures and high pressures) [1–3]. Sometimes it is called flammable ice because it burns when be inflamed [4]. In recent years, gas hydrates showed great potential in the energy supply for the industrial processes. For example, methane hydrates are widely occurring in ocean sediments around the world, and it is estimated that its energy is twice the total energy of fossil fuels [5]; so the gas hydrate process is a great opportunity for saving the energy for the future. The hydrates were recently known as the suitable new type energy saving materials due to the higher refrigerant efficiency and larger cold storage density compared to traditional storage system [6–12]. Some devices are used for measuring the thermal properties like heat capacity and latent heat of fusion of gas hydrates [13–20]. A hydrate storage system with the fusion heat near to that of ice shows the privilege of permitting the use of chilled water as circulation medium rather than antifreeze coolant or brine, which are used in ice storage systems [10,21]. Hydrate formation of water- tetrahydrofuran (THF)⁎ Corresponding author. E-mail address: [email protected] (F. Varaminian).
https://doi.org/10.1016/j.molliq.2019.111279 0167-7322/© 2019 Elsevier B.V. All rights reserved.
CO2 systems was studied by a differential scanning calorimetry (DSC) to exhibit the effect of THF on equilibrium pressure and dissociation enthalpy of CO2 hydrate [14,15]. THF and some non-gaseous molecules such as tetra-n-butylammonium Bromide (TBAB) and 1,4-dioxane can form a structure of hydrate similar to that formed by natural gas so that they can be applied as an alternative for studying the gas hydrates without the requirement of high pressures [16]. Bouchafaa and Dalmazzone found that the dissociation temperature of the mixed hydrates containing CO2 + N2, CO2 + CH4 and CO2 + H2 increases in the presence of TBAB [17]. Also, it found that TBAB-THF hydrate has the superiority for more suitable phase change temperature and increased fusion heat [18–20]. The formation of gas hydrates is a sensitive process influenced by an aqueous solution. Both thermodynamic and kinetic properties of gas hydrate are mainly affected by additives or impurities, such as surfactants, salts, and fine particulate matters [2,7,8]. Depending on the type and concentration of additives, the hydrate formation process can be facilitated or inhibited. Some researchers proved that DSC could show the effect of additives on the hydrate formation/dissociation [22–25]. McNamee used DSC technique to evaluate the hydrate nucleation and kinetic hydrate inhibitor performance trends [26]. Naeiji and Varaminian found that sodium dodecyl sulfate (SDS) and lauryl alcohol ethoxylate (LAE) lower the gas/aqueous interfacial tension and consequently increase gas diffusion into the solution in a DSC cell. Also, these surfactants improve the kinetics of CO2 hydrate formation [27]. In the next work, they observed that THF increases the kinetic parameter of methane hydrate formation N2–3 orders of magnitude [28]. It was previously reported by Parlouër et al. that sodium chloride and ethylene
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sodium halides can be an accelerator in low concentration or inhibitor in high concentration. In addition, large anions such as iodide showed to be a more effective accelerator [30]. Also, Dehghanpoor and Varaminian investigated the performance of sodium halide aqueous solutions on the kinetics of ethane hydrate formation [31]. It was found that sodium halide salts are kinetic accelerators at the concentration range of 1.5–3.1 wt%, while the rate of hydrate formation decreases at the lower and higher than that of at the range. The strength of inhibition of halide salts has a reverse relation with the atomic radius of the anions [31]. Asadi et al. examined the formation of methane gas hydrates in the presence of lithium chloride, sodium chloride, cesium chloride, sodium iodide, sodium fluoride inorganic salts. They found that a low concentration of halide sodium reduces the induction time and increases the rate of gas hydrate formation [32]. Woo et al. studied the effect of NaCl and MgCl2 on R-22 gas hydrates. Their results showed that the salts such as NaCl and MgCl2 do not participate in the gas hydrate frame-work, while the hydrate formation kinetics can be strongly influenced by the concentration of salt solution [33]. Also, Choi et al. concluded that the thermodynamic stability of HFC-125a hydrates is significantly influenced in the presence of NaCl. Due to the inhibition effect of NaCl, the equilibrium curve shifts to higher pressure regions at any given temperature. On the other hand, the gas uptake and formation rate of HFC-125a hydrates were lower at higher concentrations of NaCl because the unconverted solution was gradually enriched by NaCl as gas-hydrate formation proceeded; so, it resulted in a lower driving force for hydrate formation [34]. The above studies showed that the salts can perform as a promoter or inhibitor for hydrate formation depending on their concentration. They may shift the equilibrium curve of hydrate formation and/or affect its kinetics. As mentioned, one of interesting perspectives for hydrates is related to the hydrate storage system due to more suitable phase change temperature and increased fusion heat compared to other refrigerants. So, the measurement of the heat of hydrate formation and the effect of some additives like salts on the heat flow can be of interest in this area. In the available papers, the major predictions of the hydrate formation kinetics were made by the large-scale reactors and nonthermometric methods. So, in the present work, a new technique based on the heat released during the hydrate formation was applied to study the kinetics of the process. As well, the main goal of this study was to investigate the effect of sodium halide salts like NaF, NaCl, NaBr and NaI, and their concentrations on the kinetics of THF hydrate formation by a DSC setup.
Nomenclature A∅ Debye–Hückel constant CH4 Methane` CO2 Carbon dioxide d Differential operator DSC Differential Scanning Calorimetry H2 hydrogen gas HFC-125a Pentafluoroethane I Ionic strength LAE Lauryl Alcohol Ethoxylate m molality of ions mol% Mole percent Mw Molecular weight of water MgCl2 Magnesium chloride NaBr Sodium bromide NaCl Sodium chloride NaF Sodium fluoride NaI Sodium iodide N2 Nitrogen gas PDH Pitzer–Debye– Hückel R-22 Chlorodifluoromethane rpm Revolutions per minute SDS Sodium Dodecyl Sulfate T Temperature TBAB Tetra-n-butylammonium Bromide THF Tetrahydrofuran wt% Weight percent x Mole fraction z Charge number of ionic species Greek letters γ Activity coefficient ρ closest approach parameter Subscripts Br Bromide ion Cl Chloride ion i Component i I Iodide ion LR long-range Na Sodium ion s Salt SR short-range w Water
2. Experimental 2.1. Materials
glycol influence the methane hydrate formation so that they could shift the dissociation temperature obtained from DSC to a higher value [29]. It shows that these additives could postpone the dissociation of hydrate. Several studies were carried out regarding the effect of salts on the formation of gas hydrates [7]. Nguyen et al. studied the effect of sodium halide on the formation of methane gas hydrates [30]. They found that
The salts used in this work include sodium chloride, sodium bromide, sodium iodide, and sodium fluoride which were supplied by Merck Company. Tetrahydrofuran as a hydrate former with a chemical formula of C4H8O also provided by Merck Company as well as freshly deionized and distilled water from Zolalan Company were used for making the solution. Some properties of the used materials are presented in Table 1.
Table 1 The properties of the material used in this work. Component Tetrahydrofuran Sodium fluoride Sodium chloride Sodium bromide Sodium iodide Water
Chemical formula C4H8O NaF NaCl NaBr NaI H2O
Purity 99% N99.5% 99% 99% 99% Deionized
Ionic radius for anion (pm)
Molecular weight (g/mol)
Enthalpy of hydration for anion (kJ/mol)
Supplier
– 133 181 196 220 –
72.11 41.98 58.44 102.89 149.89 18.02
– −503 −369 −336 −298 –
Merck Merck Merck Merck Merck Zolalan
E. Hoseinynezhad, F. Varaminian / Journal of Molecular Liquids 292 (2019) 111279
2.2. Apparatus
3
Table 2 List of sensor used in this work.
The DSC device used in this work is shown schematically in Fig. 1. The device consists of a Pyrex double glazed reactor with a volume of 500 cm3, and two cylindrical Pyrex cells (reference and sample) with an internal volume of about 20 cm3 inside the reactor. Cooling of the cells was performed by a refrigerant fluid of ethylene glycol solution of 50 mol% which was filled in the reactor stirred by magnet drive and was also circulated in the jacket by a cooling system. The magnetic stirrer (Heidolpg-Hei-mixs) with a maximum rotation speed of 800 rpm was applied to provide a constant temperature for the fluid inside the reactor. The temperature of the calorimeter was controlled by the cooling system at the given value. The temperature of each cell (reference and sample) was measured by a platinum thermometer (Pt − 100) with a temperature accuracy of ±0.01 °C which was located inside the cell (see Table 2). A data acquisition system was used for recording the temperature and doing the calculation related to the calorimetry with an uncertainty of ±0.1 W for the heat flow. The device works at the atmospheric pressure and a temperature range of −15 to 25 C ̊ .
2.3. Experimental procedure Both experimental cells were washed with purified water and then dried. Afterward, 2 ± 0.1 cc of water-THF solution in the molar composition of THF.17H2 O without or with the presence of salts at different concentrations (0.15, 0.225, 0.3, 0.6, 1.2, 2.4 and 4.8 wt%) was injected into the sample cell. The sample and empty reference cells were covered with a cap and inserted in the reactor. The cooling process was operated until the temperature of the calorimeter was set at the given value. The temperature of solution into the cell was gradually reduced from 25 ̊C to −5 ̊C. The temperature changes were recorded per second by the data acquisition system. It should be noticed that firstly the calibration runs were performed based on the freezing/melting temperature and heat of fusion of ice because it is more closely with the experimental temperature range. The experimental trials were repeated two or three times to confirm the results repeatability.
Sensor
Type
Accuracy
Range
Thermocouple
Pt-100
±0.01 °C
-100~ + 400 °C
Manufacturer JUMO
3. Results and discussion Heat flow diagram of THF hydrate formation recorded by the DSC device at a constant temperature of the calorimeter of −5 ̊C is shown in Fig. 2. After the induction period, the heat is released because of the hydrate formation which appears as a wide peak. As shown, the exothermic peak is made of two parts: the rising and declining parts. The rising part which the heat flow suddenly increases continues until it reaches the metastable condition meaning at the top of peak. Afterward, the second part occurs; the hydrate formation heat is not sufficiently above that of cooling the system and so the peak reaches the base line again. Since a large amount of hydrate form at the first part, it is considered as an effective part for studying the effect of the additive on the kinetics of hydrate formation. When the salts are added to the solution, the shape of the formation peaks is changed, as shown in Fig. 3. It not only becomes narrower, but the slope of its rising part also decreases. It shows that the heat of hydrate formation reduces generally and the amount of formed hydrate decreases; as a result, the salts can inhibit the hydrate formation so that the process cannot progress easily. For evaluating the performance of different salts on the hydrate formation kinetics, the slope of its rising part is taken into account as the growth rate. Fig. 4 shows the growth rate obtained from the experimental results of tetrahydrofuran hydrate formation in the presence of sodium halide salts. As shown, the growth rate of THF hydrate is about 0.12 mol/s, while it reaches about 0.0 in the presence of salts. The reason might be that the clathrate cages have to be electrostatically neutral, but experimentally, it is seen that hydrate samples do not have net dipole moments because the disordered positions of water protons in the cage causes the lowest (about zero) dipole moment. So, the hydrate surface is positively charged by occupying the hydrogen atoms [35]. When the salts are added to the solution, the anions have a more effect on the hydrate crystalline network. These anions can
Fig. 1. Schematic of the DSC device.
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E. Hoseinynezhad, F. Varaminian / Journal of Molecular Liquids 292 (2019) 111279
0.14
2.50
NaF
0.12
NaCl NaBr
growth rate (mol/s)
Heat flow (W)
2.00
1.50
1.00
0.10
NaI
0.08 0.06 0.04
0.50
0.02 0.00 0
500
1000
1500
2000
2500
0.00
3000
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
Concentration (w%)
Time (s) Fig. 2. DSC curve of THF hydrate formation at −5 C ̊ .
Fig. 4. The growth rate of THF hydrate versus the concentration of sodium halide salts.
destroy hydrogen bonding between the water molecules and prevent to progress the hydrate formation [36]. Park et al. found that the inhibition effect of salts on the hydrate formation is directly related to the number of electrical charges, while it has an inverse relationship to the ionic radius [37]. The ionic radius of fluorine is the smallest one among the anions used in this work (as given in Table 1), however, its enthalpy of hydration is the highest. The enthalpy of hydration is a measure of the energy released when attraction occurs between the positive or negative ions and the water molecules. If the attraction is stronger, the hydration enthalpy gets bigger. Consequently, fluorine is an effective anion for disturbing the hydrate cages, so that it can significantly decrease the growth rate of hydrate, as seen in Fig. 4. On the other hand, Iodine with the largest ionic radius and the lowest enthalpy of hydration has a weak effect on the prevention of hydrate formation. It is clear from Fig. 4 that the growth rate of hydrate changes with the concentration of salts in a different manner. The results showed that the hydrate formation rate decreases with increasing the concentration of salts up to 0.6 wt%, while it increases when the concentration is at a range of 0.6 to 1.2 wt%. The rate began to decrease by the concentration of N1.2 wt%. This manner can be explained through two different viewpoints: The first viewpoint is that with increasing the concentration of salts up to 0.6 wt%, the additives intend to dissolve into the solution by THF molecules instead of water molecules. So, the amount of THF molecules in the solution for forming of hydrate becomes less than the stoichiometric value (THF.17H2O). It is expected that the hydrate cages diminish and the growth rate also decreases in this condition. While the
stronger electrostatic interaction between water molecules and ions dissolved compared to that of THF molecules and ions appears in the concentration range of 0.6 to 1.2 wt% of salts. So, the dissolved ions preferentially interact with the water molecules instead of THF molecules. As a result, the amount of THF is enough to form the hydrate cages, and then the hydrate formation condition is easily provided. In the case of the concentration of salts higher than 1.2 wt%, the number of free water molecules is lowered because the water hydration shell around the ions in the aqueous phase increases. Consequently, the progress of THF hydrate formation decreases and its kinetics becomes slower. Next viewpoint is related to the thermodynamic concepts of an electrolyte system which is more complicated than the non-electrolyte system because of existence both the ionic and molecular particles. There are three types of interactions: the interactions between ionic particles that it is naturally a long-range interaction, and interactions between molecular particles and molecular-ionic particles that both of them are a kind of short-range interactions. The long-range interaction means that the electrostatic forces do not correlate with the arrangement of molecules; That is, even if two particles with positive or negative partial charges are located far from each other, electrostatic forces can still overcome [38,39]. In the electrolyte solutions, the total activity coefficient at the high concentration of additives is the summation of the activity coefficients due to long-range (γLR) and short-range (γSR) interactions [38]: ð1Þ
Pitzer–Debye– Hückel offered a successful equation to determine the activity coefficient of the electrolyte solutions [40]:
4.00 Without Salt
3.50
0.15 wt% NaI 0.225 wt% NaI
3.00
ð lnγi ÞPDH
0.3 wt% NaI
Heat flow (W)
γ ¼ γ LR þ γ SR
2.50
0.6 wt% NaI
0 1 3 1 −2Im 2 C 2 2 1 2 1000 0:5 B 2z z I C B i ln 1 þ ρIm 2 þ i m ¼ −A∅ A @ ρ 1 Mw 1 þ ρIm 2
ð2Þ
1.2 wt% NaI
2.00
2.4 wt% NaI
1.50
4.8 wt% NaI
and
1.00
Im ¼
0.50 0.00 0
1000
2000
3000
4000
5000
6000
7000
Time (s) Fig. 3. DSC curve of THF hydrate formation in the presence of NaI at −5 C ̊ .
1 Σmi :z2i ; 2
in this work Im ¼
1 mNa þ mCl;Br;I 2
ð3Þ
where A∅ is Debye–Hückel constant, I indicates the ionic strength of the solution based on the molality of ions, Mw is molecular weight of water, m presents the molality of ions, z is the charge number of ionic species, and ρ indicates the closest approach parameter.
E. Hoseinynezhad, F. Varaminian / Journal of Molecular Liquids 292 (2019) 111279
When the salt is added to the solution, the activity coefficient of water is changed [40]. Miyawaki et al. found that the activity coefficient of water shows a more decrease in the presence of sodium chloride compared to other aqueous solutions since it is dissociated into sodium and chloride ions as a strong electrolyte [41]. So, it expects that the salt solutions used in this work affect the activity coefficient of water and consequently the hydrate formation process. The inverse correlation between the activity coefficients of water (γw) and salts solution (γs) can be shown through the Gibbs-Duhem equation, as follows: Σxi d ln γ i ¼ 0
ð4Þ
xs d lnγ s þ xw d lnγ w ¼ 0
ð5Þ
Finally, the following equation is provided by dividing both sides of Eq. (5) by dxs and then integrating: Z xs ln γ s ¼ −
xw
d lnγ w dxs −dxw
ð6Þ
So, the equation shows that the activity coefficients of water and salt solution have a reverse correlation with each other. As a result, the activity coefficient of water decreases in the presence of the anions. The activity coefficients versus the ionic strength of NaCl, NaBr, and NaI solutions obtained from the literature are shown in Fig. 5. It can be found that at the low concentration of salts, the activity of salt solutions reduces as ionic strength increases. However, some literature showed that it might increase at higher concentration. This property of the electrolyte solutions might explain the growth rate changes as a function of salts concentration. These results are shown and compared in Fig. 6.
5
As shown in Fig. 6, the activity coefficient of salts decreases with increasing the ionic strength up to about 1.7, 1 and 0.7 for NaCl, NaBr and NaI solution, respectively. According to the Gibbs-Duhem equation, the activity coefficient of water increases. So, water molecules do not intend to participate in the crystal structure due to the electrostatic force that it makes a barrier for the hydrate formation and the rate of hydrate growth also decreases. After that, the activity coefficient of salts increases with increasing the ionic strength, so that the activity coefficient of water decreases. This provides a better condition to make the crystalline structure and accelerate the process of hydrate formation. So, the activity coefficient of salts changes with their ionic strength can well explain the different manner of the hydrate growth rate with concentration of salt solutions. Unfortunately, it was not found any experimental or theoretical research related to the activity coefficient of NaF in terms of ionic strength. But, it can be predicted that the NaF solution shows a similar manner to other salts. 4. Conclusion In this work, the effect of sodium halide salts on the kinetics of THF hydrate formation was investigated. A differential scanning calorimetry setup was used to measure the heat of hydrate formation and its growth rate. According to the results, the anions of salts can disrupt hydrogen bonding between the water molecules and prevent to progress the hydrate formation. The ionic radius of fluorine is the smallest one among the anions used in this work; however, its enthalpy of hydration is the highest. This anion was the most effective material to prevent the hydrate formation. Moreover, the results showed that the growth rate of THF hydrate formation changes as a function of concentrations of sodium halide salts into the solution. The growth rate sharply decreases with increasing the salts concentration up to about 0.6 wt%, while it increases when the concentration increases from about 0.6 to 1.2 wt%. At higher concentration of
Fig. 5. Mean activity coefficient for NaCl, NaBr and NaI solution obtained from various activity models [37].
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E. Hoseinynezhad, F. Varaminian / Journal of Molecular Liquids 292 (2019) 111279
1.2
0.14 growth rate (mol/s)
NaCl
1.0
ɤs
0.8 0.6 0.4 0.2
NaCl
0.12 0.10 0.08 0.06 0.04 0.02 0.00
0.0 0
0.5
1
1.5
2
2.5
3
0
3.5
0.5
Ionic strength (mol/kg)
2
2.5
0.14 growth rate (mol/s)
NaBr
1.0 0.8
ɤs
1.5
0.6 0.4 0.2 0.0 0
0.5
1
1.5
0.10 0.08 0.06 0.04 0.02 0.00
2
0
growth rate (mol/s)
0.6 0.4 0.2 0.0 0.5
1
1
1.5
0.14
0.8
0
0.5
2
Concentration (mol/kg)
NaI
1.0
3.5
NaBr
0.12
Ionic strength (mol/kg)
1.2
3
Concentration (mol/kg)
1.2
ɤs
1
1.5
Ionic strength (mol/kg)
NaI
0.12 0.10 0.08 0.06 0.04 0.02 0.00 0
0.5
1
1.5
Concentration (mol/kg)
Fig. 6. Activity coefficient versus ionic strength [42] and growth rate versus the salts concentration of NaCl, NaBr, and NaI solutions used in this work.
salt, the formation rate of hydrate again decreases. This different manner of the growth rate with the salts concentration can be attributed to the thermo physical parameters such as the hydration effect and activity coefficients of salt solution.
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[33] Y. Woo, C. Lee, J.H. Jeong, D. Kim, J.W. Lee, Y. Yamamoto, J.H. Yoon, Clathrate hydrate formation in NaCl and MgCl2 brines at low pressure conditions, Sep. Purif. Technol. 209 (2019) 56–64. [34] W. Choi, Y. Lee, J. Mok, S. Lee, J.D. Lee, Y. Seo, Thermodynamic and kinetic influences of NaCl on HFC-125a hydrates and their significance in gas hydrate-based desalination, Chem. Eng. J. 358 (2019) 598–605. [35] A. Erfani, E. Fallah-Jokandan, F. Varaminian, Effects of non-ionic surfactants on formation kinetics of structure H hydrate regarding transportation and storage of natural gas, J. Na. Gas Sci. Eng. 37 (2017) 397–408. [36] C. Giavarini, K. Hester, The Structure and Formation of Gas Hydrates, Physical Properties of Hydrates, Hydrates Seen as a Problem for the Oil and Gas Industry, Gas Hydrates, Immense Energy Potential and Environmental Challenges, Springer-Verlag, London, 2011. [37] K.N. Park, S.Y. Hong, J.W. Lee, K.C. Kang, Y.C. Lee, M.G. Ha, A new apparatus for seawater desalination by gas hydrate process and removal characteristics of dissolved minerals (Na+, Mg2+, Ca2+, K+, B3+), Desalination 274 (2011) 91–96. [38] H. Ahmadi, K. Peyvandi, Electrolyte-UNIQUAC-NRF model based on ion specific parameters for the correlation of mean activity coefficients of electrolyte solutions, J. Solut. Chem. 46 (2017) 1202–1219. [39] A. Haghtalab, K. Peyvandi, Electrolyte-UNIQUAC-NRF model for the correlation of the mean activity coefficient of electrolyte solutions, Fluid Phase Equilib. 281 (2009) 163–171. [40] K.S. Pitzer, Electrolytes. From dilute solution to fused salts, J. Am. Chem. Soc. 102 (1980) 2902–2906. [41] O. Miyawaki, A. Saito, T. Matsuo, K. Nakamura, Activity and activity coefficient of water in aqueous solutions and their relationships with solution structure parameters, Biosci. Biotechnol. Biochem. 61 (1997) 466–469. [42] M.N. Khan, P. Warrier, C.J. Peters, C.A. Koh, Mean activity coefficient of electrolytes: a critical evaluation of four physical models, J. Nat. Gas Sci. Eng. 35 (2016) 1355–1361.
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
The effect of sodium halide salts on the kinetics of tetrahydrofuran hydrate formation by using a differential scanning calorimetry method Elnaz Hoseinynezhad, Farshad Varaminian ⁎ Faculty of Chemical, Gas and Petroleum Engineering, Semnan University, Semnan, Iran
a r t i c l e
i n f o
Article history: Received 26 December 2018 Received in revised form 16 June 2019 Accepted 30 June 2019 Available online 2 July 2019 Keywords: Sodium halides Tetrahydrofuran Differential scanning calorimetry Formation kinetics Gas hydrate
a b s t r a c t In this study, the effect of sodium halide salts including Sodium Fluoride, Sodium Iodide, Sodium Bromide, and Sodium Chloride was investigated on the kinetics of tetrahydrofuran hydrate formation by using a differential scanning calorimetry setup. It was found that the anions of salts can destroy hydrogen bonding between the water molecules and prevent the growth of gas hydrate. The fluorine with the smallest ionic radius and the highest enthalpy of hydration was the most effective material to decrease the hydrate growth rate. According to the results, the formation kinetics of hydrate changes with the concentration of salts in a different manner. Results show that the hydrate formation rate decreases with increasing the concentration of salts up to 0.6 wt%, while it increases when the concentration is at a range of 0.6 to 1.2 wt%. The rate begins to decrease by the concentration of N1.2 wt%. It was found that the parameters such as the hydration effect and the activity coefficients of salt solutions can affect the kinetics of tetrahydrofuran hydrate formation. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Natural gas hydrates are known as one of the most effective alternatives to oil resources [1–3]. Governmental agencies at around the world have supported the study and development of the hydrate resources so that they could provide a great achievement in theoretical researches, applied techniques and equipment innovations. Natural gas hydrate is a crystalline compound that formed through water and gas molecules at the specific conditions (low temperatures and high pressures) [1–3]. Sometimes it is called flammable ice because it burns when be inflamed [4]. In recent years, gas hydrates showed great potential in the energy supply for the industrial processes. For example, methane hydrates are widely occurring in ocean sediments around the world, and it is estimated that its energy is twice the total energy of fossil fuels [5]; so the gas hydrate process is a great opportunity for saving the energy for the future. The hydrates were recently known as the suitable new type energy saving materials due to the higher refrigerant efficiency and larger cold storage density compared to traditional storage system [6–12]. Some devices are used for measuring the thermal properties like heat capacity and latent heat of fusion of gas hydrates [13–20]. A hydrate storage system with the fusion heat near to that of ice shows the privilege of permitting the use of chilled water as circulation medium rather than antifreeze coolant or brine, which are used in ice storage systems [10,21]. Hydrate formation of water- tetrahydrofuran (THF)⁎ Corresponding author. E-mail address: [email protected] (F. Varaminian).
https://doi.org/10.1016/j.molliq.2019.111279 0167-7322/© 2019 Elsevier B.V. All rights reserved.
CO2 systems was studied by a differential scanning calorimetry (DSC) to exhibit the effect of THF on equilibrium pressure and dissociation enthalpy of CO2 hydrate [14,15]. THF and some non-gaseous molecules such as tetra-n-butylammonium Bromide (TBAB) and 1,4-dioxane can form a structure of hydrate similar to that formed by natural gas so that they can be applied as an alternative for studying the gas hydrates without the requirement of high pressures [16]. Bouchafaa and Dalmazzone found that the dissociation temperature of the mixed hydrates containing CO2 + N2, CO2 + CH4 and CO2 + H2 increases in the presence of TBAB [17]. Also, it found that TBAB-THF hydrate has the superiority for more suitable phase change temperature and increased fusion heat [18–20]. The formation of gas hydrates is a sensitive process influenced by an aqueous solution. Both thermodynamic and kinetic properties of gas hydrate are mainly affected by additives or impurities, such as surfactants, salts, and fine particulate matters [2,7,8]. Depending on the type and concentration of additives, the hydrate formation process can be facilitated or inhibited. Some researchers proved that DSC could show the effect of additives on the hydrate formation/dissociation [22–25]. McNamee used DSC technique to evaluate the hydrate nucleation and kinetic hydrate inhibitor performance trends [26]. Naeiji and Varaminian found that sodium dodecyl sulfate (SDS) and lauryl alcohol ethoxylate (LAE) lower the gas/aqueous interfacial tension and consequently increase gas diffusion into the solution in a DSC cell. Also, these surfactants improve the kinetics of CO2 hydrate formation [27]. In the next work, they observed that THF increases the kinetic parameter of methane hydrate formation N2–3 orders of magnitude [28]. It was previously reported by Parlouër et al. that sodium chloride and ethylene
2
E. Hoseinynezhad, F. Varaminian / Journal of Molecular Liquids 292 (2019) 111279
sodium halides can be an accelerator in low concentration or inhibitor in high concentration. In addition, large anions such as iodide showed to be a more effective accelerator [30]. Also, Dehghanpoor and Varaminian investigated the performance of sodium halide aqueous solutions on the kinetics of ethane hydrate formation [31]. It was found that sodium halide salts are kinetic accelerators at the concentration range of 1.5–3.1 wt%, while the rate of hydrate formation decreases at the lower and higher than that of at the range. The strength of inhibition of halide salts has a reverse relation with the atomic radius of the anions [31]. Asadi et al. examined the formation of methane gas hydrates in the presence of lithium chloride, sodium chloride, cesium chloride, sodium iodide, sodium fluoride inorganic salts. They found that a low concentration of halide sodium reduces the induction time and increases the rate of gas hydrate formation [32]. Woo et al. studied the effect of NaCl and MgCl2 on R-22 gas hydrates. Their results showed that the salts such as NaCl and MgCl2 do not participate in the gas hydrate frame-work, while the hydrate formation kinetics can be strongly influenced by the concentration of salt solution [33]. Also, Choi et al. concluded that the thermodynamic stability of HFC-125a hydrates is significantly influenced in the presence of NaCl. Due to the inhibition effect of NaCl, the equilibrium curve shifts to higher pressure regions at any given temperature. On the other hand, the gas uptake and formation rate of HFC-125a hydrates were lower at higher concentrations of NaCl because the unconverted solution was gradually enriched by NaCl as gas-hydrate formation proceeded; so, it resulted in a lower driving force for hydrate formation [34]. The above studies showed that the salts can perform as a promoter or inhibitor for hydrate formation depending on their concentration. They may shift the equilibrium curve of hydrate formation and/or affect its kinetics. As mentioned, one of interesting perspectives for hydrates is related to the hydrate storage system due to more suitable phase change temperature and increased fusion heat compared to other refrigerants. So, the measurement of the heat of hydrate formation and the effect of some additives like salts on the heat flow can be of interest in this area. In the available papers, the major predictions of the hydrate formation kinetics were made by the large-scale reactors and nonthermometric methods. So, in the present work, a new technique based on the heat released during the hydrate formation was applied to study the kinetics of the process. As well, the main goal of this study was to investigate the effect of sodium halide salts like NaF, NaCl, NaBr and NaI, and their concentrations on the kinetics of THF hydrate formation by a DSC setup.
Nomenclature A∅ Debye–Hückel constant CH4 Methane` CO2 Carbon dioxide d Differential operator DSC Differential Scanning Calorimetry H2 hydrogen gas HFC-125a Pentafluoroethane I Ionic strength LAE Lauryl Alcohol Ethoxylate m molality of ions mol% Mole percent Mw Molecular weight of water MgCl2 Magnesium chloride NaBr Sodium bromide NaCl Sodium chloride NaF Sodium fluoride NaI Sodium iodide N2 Nitrogen gas PDH Pitzer–Debye– Hückel R-22 Chlorodifluoromethane rpm Revolutions per minute SDS Sodium Dodecyl Sulfate T Temperature TBAB Tetra-n-butylammonium Bromide THF Tetrahydrofuran wt% Weight percent x Mole fraction z Charge number of ionic species Greek letters γ Activity coefficient ρ closest approach parameter Subscripts Br Bromide ion Cl Chloride ion i Component i I Iodide ion LR long-range Na Sodium ion s Salt SR short-range w Water
2. Experimental 2.1. Materials
glycol influence the methane hydrate formation so that they could shift the dissociation temperature obtained from DSC to a higher value [29]. It shows that these additives could postpone the dissociation of hydrate. Several studies were carried out regarding the effect of salts on the formation of gas hydrates [7]. Nguyen et al. studied the effect of sodium halide on the formation of methane gas hydrates [30]. They found that
The salts used in this work include sodium chloride, sodium bromide, sodium iodide, and sodium fluoride which were supplied by Merck Company. Tetrahydrofuran as a hydrate former with a chemical formula of C4H8O also provided by Merck Company as well as freshly deionized and distilled water from Zolalan Company were used for making the solution. Some properties of the used materials are presented in Table 1.
Table 1 The properties of the material used in this work. Component Tetrahydrofuran Sodium fluoride Sodium chloride Sodium bromide Sodium iodide Water
Chemical formula C4H8O NaF NaCl NaBr NaI H2O
Purity 99% N99.5% 99% 99% 99% Deionized
Ionic radius for anion (pm)
Molecular weight (g/mol)
Enthalpy of hydration for anion (kJ/mol)
Supplier
– 133 181 196 220 –
72.11 41.98 58.44 102.89 149.89 18.02
– −503 −369 −336 −298 –
Merck Merck Merck Merck Merck Zolalan
E. Hoseinynezhad, F. Varaminian / Journal of Molecular Liquids 292 (2019) 111279
2.2. Apparatus
3
Table 2 List of sensor used in this work.
The DSC device used in this work is shown schematically in Fig. 1. The device consists of a Pyrex double glazed reactor with a volume of 500 cm3, and two cylindrical Pyrex cells (reference and sample) with an internal volume of about 20 cm3 inside the reactor. Cooling of the cells was performed by a refrigerant fluid of ethylene glycol solution of 50 mol% which was filled in the reactor stirred by magnet drive and was also circulated in the jacket by a cooling system. The magnetic stirrer (Heidolpg-Hei-mixs) with a maximum rotation speed of 800 rpm was applied to provide a constant temperature for the fluid inside the reactor. The temperature of the calorimeter was controlled by the cooling system at the given value. The temperature of each cell (reference and sample) was measured by a platinum thermometer (Pt − 100) with a temperature accuracy of ±0.01 °C which was located inside the cell (see Table 2). A data acquisition system was used for recording the temperature and doing the calculation related to the calorimetry with an uncertainty of ±0.1 W for the heat flow. The device works at the atmospheric pressure and a temperature range of −15 to 25 C ̊ .
2.3. Experimental procedure Both experimental cells were washed with purified water and then dried. Afterward, 2 ± 0.1 cc of water-THF solution in the molar composition of THF.17H2 O without or with the presence of salts at different concentrations (0.15, 0.225, 0.3, 0.6, 1.2, 2.4 and 4.8 wt%) was injected into the sample cell. The sample and empty reference cells were covered with a cap and inserted in the reactor. The cooling process was operated until the temperature of the calorimeter was set at the given value. The temperature of solution into the cell was gradually reduced from 25 ̊C to −5 ̊C. The temperature changes were recorded per second by the data acquisition system. It should be noticed that firstly the calibration runs were performed based on the freezing/melting temperature and heat of fusion of ice because it is more closely with the experimental temperature range. The experimental trials were repeated two or three times to confirm the results repeatability.
Sensor
Type
Accuracy
Range
Thermocouple
Pt-100
±0.01 °C
-100~ + 400 °C
Manufacturer JUMO
3. Results and discussion Heat flow diagram of THF hydrate formation recorded by the DSC device at a constant temperature of the calorimeter of −5 ̊C is shown in Fig. 2. After the induction period, the heat is released because of the hydrate formation which appears as a wide peak. As shown, the exothermic peak is made of two parts: the rising and declining parts. The rising part which the heat flow suddenly increases continues until it reaches the metastable condition meaning at the top of peak. Afterward, the second part occurs; the hydrate formation heat is not sufficiently above that of cooling the system and so the peak reaches the base line again. Since a large amount of hydrate form at the first part, it is considered as an effective part for studying the effect of the additive on the kinetics of hydrate formation. When the salts are added to the solution, the shape of the formation peaks is changed, as shown in Fig. 3. It not only becomes narrower, but the slope of its rising part also decreases. It shows that the heat of hydrate formation reduces generally and the amount of formed hydrate decreases; as a result, the salts can inhibit the hydrate formation so that the process cannot progress easily. For evaluating the performance of different salts on the hydrate formation kinetics, the slope of its rising part is taken into account as the growth rate. Fig. 4 shows the growth rate obtained from the experimental results of tetrahydrofuran hydrate formation in the presence of sodium halide salts. As shown, the growth rate of THF hydrate is about 0.12 mol/s, while it reaches about 0.0 in the presence of salts. The reason might be that the clathrate cages have to be electrostatically neutral, but experimentally, it is seen that hydrate samples do not have net dipole moments because the disordered positions of water protons in the cage causes the lowest (about zero) dipole moment. So, the hydrate surface is positively charged by occupying the hydrogen atoms [35]. When the salts are added to the solution, the anions have a more effect on the hydrate crystalline network. These anions can
Fig. 1. Schematic of the DSC device.
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E. Hoseinynezhad, F. Varaminian / Journal of Molecular Liquids 292 (2019) 111279
0.14
2.50
NaF
0.12
NaCl NaBr
growth rate (mol/s)
Heat flow (W)
2.00
1.50
1.00
0.10
NaI
0.08 0.06 0.04
0.50
0.02 0.00 0
500
1000
1500
2000
2500
0.00
3000
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
Concentration (w%)
Time (s) Fig. 2. DSC curve of THF hydrate formation at −5 C ̊ .
Fig. 4. The growth rate of THF hydrate versus the concentration of sodium halide salts.
destroy hydrogen bonding between the water molecules and prevent to progress the hydrate formation [36]. Park et al. found that the inhibition effect of salts on the hydrate formation is directly related to the number of electrical charges, while it has an inverse relationship to the ionic radius [37]. The ionic radius of fluorine is the smallest one among the anions used in this work (as given in Table 1), however, its enthalpy of hydration is the highest. The enthalpy of hydration is a measure of the energy released when attraction occurs between the positive or negative ions and the water molecules. If the attraction is stronger, the hydration enthalpy gets bigger. Consequently, fluorine is an effective anion for disturbing the hydrate cages, so that it can significantly decrease the growth rate of hydrate, as seen in Fig. 4. On the other hand, Iodine with the largest ionic radius and the lowest enthalpy of hydration has a weak effect on the prevention of hydrate formation. It is clear from Fig. 4 that the growth rate of hydrate changes with the concentration of salts in a different manner. The results showed that the hydrate formation rate decreases with increasing the concentration of salts up to 0.6 wt%, while it increases when the concentration is at a range of 0.6 to 1.2 wt%. The rate began to decrease by the concentration of N1.2 wt%. This manner can be explained through two different viewpoints: The first viewpoint is that with increasing the concentration of salts up to 0.6 wt%, the additives intend to dissolve into the solution by THF molecules instead of water molecules. So, the amount of THF molecules in the solution for forming of hydrate becomes less than the stoichiometric value (THF.17H2O). It is expected that the hydrate cages diminish and the growth rate also decreases in this condition. While the
stronger electrostatic interaction between water molecules and ions dissolved compared to that of THF molecules and ions appears in the concentration range of 0.6 to 1.2 wt% of salts. So, the dissolved ions preferentially interact with the water molecules instead of THF molecules. As a result, the amount of THF is enough to form the hydrate cages, and then the hydrate formation condition is easily provided. In the case of the concentration of salts higher than 1.2 wt%, the number of free water molecules is lowered because the water hydration shell around the ions in the aqueous phase increases. Consequently, the progress of THF hydrate formation decreases and its kinetics becomes slower. Next viewpoint is related to the thermodynamic concepts of an electrolyte system which is more complicated than the non-electrolyte system because of existence both the ionic and molecular particles. There are three types of interactions: the interactions between ionic particles that it is naturally a long-range interaction, and interactions between molecular particles and molecular-ionic particles that both of them are a kind of short-range interactions. The long-range interaction means that the electrostatic forces do not correlate with the arrangement of molecules; That is, even if two particles with positive or negative partial charges are located far from each other, electrostatic forces can still overcome [38,39]. In the electrolyte solutions, the total activity coefficient at the high concentration of additives is the summation of the activity coefficients due to long-range (γLR) and short-range (γSR) interactions [38]: ð1Þ
Pitzer–Debye– Hückel offered a successful equation to determine the activity coefficient of the electrolyte solutions [40]:
4.00 Without Salt
3.50
0.15 wt% NaI 0.225 wt% NaI
3.00
ð lnγi ÞPDH
0.3 wt% NaI
Heat flow (W)
γ ¼ γ LR þ γ SR
2.50
0.6 wt% NaI
0 1 3 1 −2Im 2 C 2 2 1 2 1000 0:5 B 2z z I C B i ln 1 þ ρIm 2 þ i m ¼ −A∅ A @ ρ 1 Mw 1 þ ρIm 2
ð2Þ
1.2 wt% NaI
2.00
2.4 wt% NaI
1.50
4.8 wt% NaI
and
1.00
Im ¼
0.50 0.00 0
1000
2000
3000
4000
5000
6000
7000
Time (s) Fig. 3. DSC curve of THF hydrate formation in the presence of NaI at −5 C ̊ .
1 Σmi :z2i ; 2
in this work Im ¼
1 mNa þ mCl;Br;I 2
ð3Þ
where A∅ is Debye–Hückel constant, I indicates the ionic strength of the solution based on the molality of ions, Mw is molecular weight of water, m presents the molality of ions, z is the charge number of ionic species, and ρ indicates the closest approach parameter.
E. Hoseinynezhad, F. Varaminian / Journal of Molecular Liquids 292 (2019) 111279
When the salt is added to the solution, the activity coefficient of water is changed [40]. Miyawaki et al. found that the activity coefficient of water shows a more decrease in the presence of sodium chloride compared to other aqueous solutions since it is dissociated into sodium and chloride ions as a strong electrolyte [41]. So, it expects that the salt solutions used in this work affect the activity coefficient of water and consequently the hydrate formation process. The inverse correlation between the activity coefficients of water (γw) and salts solution (γs) can be shown through the Gibbs-Duhem equation, as follows: Σxi d ln γ i ¼ 0
ð4Þ
xs d lnγ s þ xw d lnγ w ¼ 0
ð5Þ
Finally, the following equation is provided by dividing both sides of Eq. (5) by dxs and then integrating: Z xs ln γ s ¼ −
xw
d lnγ w dxs −dxw
ð6Þ
So, the equation shows that the activity coefficients of water and salt solution have a reverse correlation with each other. As a result, the activity coefficient of water decreases in the presence of the anions. The activity coefficients versus the ionic strength of NaCl, NaBr, and NaI solutions obtained from the literature are shown in Fig. 5. It can be found that at the low concentration of salts, the activity of salt solutions reduces as ionic strength increases. However, some literature showed that it might increase at higher concentration. This property of the electrolyte solutions might explain the growth rate changes as a function of salts concentration. These results are shown and compared in Fig. 6.
5
As shown in Fig. 6, the activity coefficient of salts decreases with increasing the ionic strength up to about 1.7, 1 and 0.7 for NaCl, NaBr and NaI solution, respectively. According to the Gibbs-Duhem equation, the activity coefficient of water increases. So, water molecules do not intend to participate in the crystal structure due to the electrostatic force that it makes a barrier for the hydrate formation and the rate of hydrate growth also decreases. After that, the activity coefficient of salts increases with increasing the ionic strength, so that the activity coefficient of water decreases. This provides a better condition to make the crystalline structure and accelerate the process of hydrate formation. So, the activity coefficient of salts changes with their ionic strength can well explain the different manner of the hydrate growth rate with concentration of salt solutions. Unfortunately, it was not found any experimental or theoretical research related to the activity coefficient of NaF in terms of ionic strength. But, it can be predicted that the NaF solution shows a similar manner to other salts. 4. Conclusion In this work, the effect of sodium halide salts on the kinetics of THF hydrate formation was investigated. A differential scanning calorimetry setup was used to measure the heat of hydrate formation and its growth rate. According to the results, the anions of salts can disrupt hydrogen bonding between the water molecules and prevent to progress the hydrate formation. The ionic radius of fluorine is the smallest one among the anions used in this work; however, its enthalpy of hydration is the highest. This anion was the most effective material to prevent the hydrate formation. Moreover, the results showed that the growth rate of THF hydrate formation changes as a function of concentrations of sodium halide salts into the solution. The growth rate sharply decreases with increasing the salts concentration up to about 0.6 wt%, while it increases when the concentration increases from about 0.6 to 1.2 wt%. At higher concentration of
Fig. 5. Mean activity coefficient for NaCl, NaBr and NaI solution obtained from various activity models [37].
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E. Hoseinynezhad, F. Varaminian / Journal of Molecular Liquids 292 (2019) 111279
1.2
0.14 growth rate (mol/s)
NaCl
1.0
ɤs
0.8 0.6 0.4 0.2
NaCl
0.12 0.10 0.08 0.06 0.04 0.02 0.00
0.0 0
0.5
1
1.5
2
2.5
3
0
3.5
0.5
Ionic strength (mol/kg)
2
2.5
0.14 growth rate (mol/s)
NaBr
1.0 0.8
ɤs
1.5
0.6 0.4 0.2 0.0 0
0.5
1
1.5
0.10 0.08 0.06 0.04 0.02 0.00
2
0
growth rate (mol/s)
0.6 0.4 0.2 0.0 0.5
1
1
1.5
0.14
0.8
0
0.5
2
Concentration (mol/kg)
NaI
1.0
3.5
NaBr
0.12
Ionic strength (mol/kg)
1.2
3
Concentration (mol/kg)
1.2
ɤs
1
1.5
Ionic strength (mol/kg)
NaI
0.12 0.10 0.08 0.06 0.04 0.02 0.00 0
0.5
1
1.5
Concentration (mol/kg)
Fig. 6. Activity coefficient versus ionic strength [42] and growth rate versus the salts concentration of NaCl, NaBr, and NaI solutions used in this work.
salt, the formation rate of hydrate again decreases. This different manner of the growth rate with the salts concentration can be attributed to the thermo physical parameters such as the hydration effect and activity coefficients of salt solution.
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